In 1991, Bobby Bishop and his father were operating a skydiving operation out of Celina, Texas. They had a Cessna 182, a Pilatus PC-6 Porter, a DC-3 and a de Havilland Caribou. But they wanted something in between the Porter and the DC-3/Caribou size aircraft. A de Havilland Twin Otter didn't seem cost-effective, and the single-engine Otter — even one with a 750 shaft-horsepower (shp) conversion — didn't have quite the power they wanted.
"We said, 'What we really need is an Otter with a 1,000 hp engine,'" Bishop remembers.
So Bishop and his dad did the work to get a Supplemental Type Certificate (STC) for replacing the Otter's stock 600 hp Pratt & Whitney 1340 radial engine with a flat-rated, 900 shp Honeywell TPE331-10 turbine engine.
"We figured that even if we never did it again, we could use it in our skydiving operation," Bishop says. But very soon "the Alaska guys heard about this Otter with this huge engine down in Texas," and the phone started to ring.
At the time, the Bishops had gotten only a single-use STC, but an Alaskan bush pilot named Paul Claus sent Bishop, sight unseen, a half-million dollars to do a similar conversion for him. So the Bishops got a multiple-use STC … and found themselves in a new line of work.
Eighteen years later, Texas Turbine Conversions, the company the Bishops founded to do their conversion work, is a thriving business that performs turbine upgrades for both Otters and Cessna Caravans. (The Caravan conversion replaces the stock Pratt & Whitney PT6A-114 600 shp or PT6A-114A 675 shp engine with a flat-rated 900 shp Honeywell TPE331-12JR turboprop engine.)
Of course, Texas Turbines is far from the only engine conversion operation out there. Ever since the Wright brothers figured they could build a better engine if they built one themselves, aviation manufacturers and after-market shops have been hard at work figuring out how to make a better mousetrap. Sometimes, a new engine is simply matched with a new airframe on the production line. But often, even if that happens, inquiring minds soon start thinking about how much better an older model of that airplane would perform if it just had that upgraded engine in it.
In the piston market, any number of individuals modify existing engines into experimental models with different carburetors, bigger pistons or other power-producing modifications. But fewer turbine owners are open to the idea of an experimental engine or airplane. So most turbine "modifications" consist of either an engine manufacturer making certified upgrades to an existing engine, or a shop replacing an older engine with a more capable, newer model — but with an FAA-approved STC, so the aircraft is still a fully certified, insurable product.
Turbine conversions and upgrades are done by both original engine manufacturers, such as Pratt & Whitney and Honeywell, and individual, after-market shops such as Texas Turbines. The individual after-market shops tend to specialize in a few specific modifications for which they own the STC. They also tend to specialize in smaller engines and aircraft, from turboprops to smaller business jets — in part because the cost of obtaining an STC on a Gulfstream V engine would be prohibitively expensive for a small, privately owned company.
At the bottom of the market are shops that convert pistons to turbines. The Otter conversions are now a minor point for Texas Turbines, but there are other shops that offer piston-to-turbine conversions. Soloy, for example, offers certified turbine conversions for 206/207 aircraft, and O&N offers the "Silver Eagle" turbine conversion for the Cessna 210.
A Soloy "Mark II" conversion, which puts a 450 shp Rolls Royce 250 series engine on a Cessna 206, costs about $650,000, but Soloy CEO Dave Stauffer points out that it still offers owners "the lowest-cost single turboprop you can buy." Soloy also offers a number of turbine helicopter conversions, including one that replaces the stock Turbomeca engine on an Aerospatiale AS350 with a Lycoming LTS101 powerplant.
Next up the ladder are the mostly independent shops that offer retrofits for existing turboprop aircraft. One of the most popular, in terms of sheer numbers, is the Blackhawk Modifications Inc. conversion program for older Beechcraft C90 and B200 King Air turboprops. Blackhawk replaces the original Pratt & Whitney engines with newer-model Pratt & Whitney engines. While the new engines are flat-rated to the same horsepower as the old ones, they're capable of holding that rated horsepower to a higher altitude.
"If you increase the amount of horsepower you put on an airframe," explains Edwin Black, director of marketing and sales for Blackhawk, "you have to recertify the airframe as well. So we keep the horsepower the same but flat-rate the engines so the climb and cruise performance are better."
On the other hand, Blackhawk is now in the process of certifying a conversion for the Cessna Caravan that would increase its horsepower by replacing the stock Pratt & Whitney PT6A-114A 675 shp engine with a Pratt & Whitney PT6A-42A engine capable of 850 horsepower.